"Wandering" critters that are unrestrained will prevent others from entering the map. I think this applies to all wandering areas separately - surface critters block other surface critters and presumably critters from cavern one other cavern-one creatures. I've only really observed it with surface creatures, though.
In one case, i had dumped a few winged animalmen down a chute closed with a locked hatch and they just hovered under the hatch forever. Nothing new spawned on the surface, which served me fine, i disliked the mess that always generated in my dozens of weapon traps.
***
Year twenty-five in Ustanoddom, "Blamelesscloister" the shining transparent fort on the glacier. A mere three children were born last year, population's at 61; still nineteen to go until we can get sieges. At the current rate, that's between six and seven years. Minecart collisions are on the way to overtake large serrated discs in the leading causes of death among local goblins. No goblin so far has made it past the third cart. In fact, only one has made it _to_ the third cart out of six. The second cart in line has scored 19 kills so far. The primary cart, dubbed Christine, a masterwork nickel silver minecart, now loaded with five native gold boulders for an in-operation weight of ten tons instead of the empty quarter ton, has soared to an impressive 85.
Thanks to the restructuring of defences, much fewer goblins get captured, so i decided to no longer keep them all stuffed into a single aquarium (which didn't work properly in a glacier, all the water put inside freezes into loose ice) and set up a proper red-cross approved goblin prison:
Below the fully occupied cells, above the buildings included.
- solitary confinement is a kind of torture, thus should be avoided. These are communal cells holding ten prisoners.
- basic amenities are provided. Each prisoner has access to a bed and to a chair with adjacent table.
- prisoners are kept safely away from harm. The cells are far out of range of trigger-happy caravan guards and the steel door allowing access is opened via a lever in the ducal rooms, and the lever is personalised so that only the fort's own military officers may use it.
Unfortunately, cultural taboos prevent dwarfs from offering and goblins from accepting medical assistance. One prisoner has died from infection after injuries suffered in our traps. Another prisoner is missing a leg (well, _he_ may be missing it, we know exactly where it is) but refuses treatment.
One cell has a holding capacity of ten, a full block of forty prisoners. Currently, i've only filled one block and one cell. We have fifty-three prisoners.